Biomagnification

Chlorinated hydrocarbons such as the insecticides DDT, DDD, dieldrin, and methoxychlor, the dielectric fluids known as PCBs, and the chlorinated dioxin, TCDD, have a very sparse solubility in water. As a result, these chemicals cannot be "diluted" into this ubiquitous solvent, which is so abundant on the surface of Earth and in organisms. Therefore, even situations considered to be highly contaminated by chlorinated hydrocarbons have very small concentrations of these chemicals in water, typically less than 1 ppb, and in the case of the dioxin TCDD smaller than 1 ppt (i.e., 0.001 ppb). However, chlorinated hydrocarbons are highly soluble in lipids. Because most lipids within ecosystems occur in biological tissues, the chlorinated hydrocarbons have a strong affinity for living organisms, and they tend to biomagnify by many orders of magnitude from vanishingly small aqueous concentrations. Furthermore, because chlorinated hydrocarbons are persistent in the environment, they accumulate progressively as organisms grow older, and they accumulate into especially large concentrations in top predators, as described previously. In some cases, older individuals of top-predator animals such as raptorial birds and fish-eating marine mammals have been found to have thousands of ppm of DDT and PCBs in their fatty tissues. The toxicity caused by these animals accumulated exposures to DDT, PCBs, and other chlorinated hydrocarbons is a well-recognized environmental problem.

The biomagnification and food-web accumulation characteristics of DDT are especially well known. Typically, DDT has extremely small concentrations in air and water, and, to a lesser degree in soil. However, concentrations are much larger in organisms, especially in animals at or near the top of their food web, such as humans and predatory birds. The food-web biomagnification of DDT can be illustrated by the case of Lake Kariba, Zimbabwe. Although banned in most industrialized countries since the early 1970s, DDT is still used in many tropical countries for agriculture purposes and to control insect vectors of human diseases. The use of DDT in agriculture was banned in Zimbabwe in 1982, but DDT continues to be used to control mosquitoes and tsetse flies, insects that spread malaria and diseases of livestock. The concentration of DDT in the water of Lake Kariba was less than 0.002 ppb, but concentrations in sediment were 0.4 ppm (because sediment contains a relatively large concentration of organic matter, it contains much more DDT than the overlying water). Planktonic algae contained 2.5 ppm. A filter-feeding mussel had 10 ppm (values for animal tissues are for DDT in fat), while two species of plant-eating fish contained 2 ppm, and a bottom-feeding fish contained 6 ppm. A predatory fish and a fish-eating bird, the great cormorant, contained 5-10 ppm. The Nile crocodile is the top predator in Lake Kariba (other than humans), and it had 34 ppm. Therefore, the data for Lake Kariba illustrates a substantial biomagnification of DDT from water, and to a lesser degree from sediment, as well as a marked food-web accumulation from herbivores to top carnivores.

The widespread occurrence of food-web biomagnification of DDT and other chlorinated hydrocarbons caused chronic, ecotoxicological damage to birds and mammals of many species, even in habitats remote from sprayed sites. In some species, effects on predatory birds were severe enough to cause large declines in abundance beginning in the early 1950s, and resulting in local or regional losses of populations. Prominent examples of North American birds that suffered population decreases because of exposure to chlorinated hydrocarbons include bald eagle, golden eagle, peregrine falcon, osprey, brown pelican, and double-crested cormorant, among others. However, since the banning of the use of DDT in North America in the early 1970s, these birds have increased in abundance. In the case of the peregrine falcon, this increase was enhanced by a captive-breeding and release program over much of its former range in eastern North America.